Before the invention is described, some relevant technical concepts will be defined.
The movements of the nozzles of a tank washing machine can generally be described as a periodic rotating movement in a plane while that plane itself is revolved around an axis which makes an angle with the axis of rotation of the nozzle. As far as is known, in all known machines the two axes of rotation are mutually perpendicular. The machines in which the two rotating movements are uniform and rotate completely, are known as so-called "Butterworth" machines In other machines the rotating movement of the spray nozzles in the plane is not uniform and completely rotating but covers only a portion of the circle and can be described as a backward and forward movement. Examples thereof are the so-called "bottom washers" and some "single nozzle machines".
Although the aforementioned axes of rotation which define the movements of the nozzle can be disposed in any desired position in the space and the movements can be such that, if desired, any portion of the space or the entire space can be covered by the jets of cleaning agent, for the sake of simplicity reference will be made to a horizontal axis which nozzles rotate about uniformly and completely, and a vertical axis which the vertical plane of rotation of the nozzles revolves around uniformly. The respective rates of rotation are designated .OMEGA..sub.h and .OMEGA..sub.v.
Although, further, any drive of rotation can be used for rotating one or more nozzles around two or more axes, hereinafter reference will be made only to a (fixed) bevel gear with a vertical axis (number of teeth=N.sub.f), over which rolls, as a planet gear, a (moving) bevel gear with a horizontal axis (number of teeth N.sub.m). In the case where the transmission ratio between the two gear nozzle rotations is in actual fact effected by means of gears, the relation between "horizontal" and "vertical" rotation can be described as: EQU T.sub.v T.sub.h =.OMEGA..sub.h /Q.sub.v =N.sub.f /N.sub.m
wherein T.sub.v and T.sub.h represent the period (period of oscillation or time of revolution) of the movements for the vertical and the horizontal axis of rotation, respectively.
The trajectory of jet impingement of one nozzle in one revolution about the horizontal axis is called a track. The width of the area cleaned by the nozzle jet is dependent upon many factors, such as distance, angle of incidence, nature and adhesion of the material to be removed to the tank wall, etc.
Due to the simultaneous rotation of the nozzle around two axes, the beginning and the end of each track, to be defined as intersections of a closed circumferential line on the tank wall, chosen as a reference, will have shifted relatively to each other. The extent of the shift depends on T.sub.v / T.sub.h.
Depending on the number of nozzles N.sub.noz, after a number of shifts the washing pattern will have made one complete round along the closed reference line and the first subsequent intersection of a nozzle jet track will occur beyond the intersection that was the first to be defined. Then one subcycle has been completed. A full cycle has been completed when after a number of rounds N.sub.track the last intersection coincides with the first. When the intersections are provided so close to each other that the shift along the closed reference line is approximately equal to the width of the jet impingement trajectory, theoretically one single cycle will suffice. In practice a complete cycle is built up from a number of subcycles, the intersections of the tracks at the closed reference line having shifted a little in each successive subcycle over a distance which is so much smaller than the distance between successive intersections in the preceding subcycle that this distance can be bridged in a number of steps in one direction. In other words, in the first subcycle a track pattern is created which is gradually densified from one side. Only after a complete cycle has been completed is a track pattern obtained of a certain density and uniformly distributed over the interior tank wall, in which the impingement tracks can overlap laterally.
Therefore, it is a drawback of the known tank cleaning method that only after a relatively long washing time a uniform dense impingement track pattern is provided so that in the case of premature interruption of the cleaning process only a small fraction of the contaminations have been removed from the tank wall.